Adaptive filters are often used in practical implementations due to their excellent performance, especially in an environment of unknown statistics or in inherently non-stationary environment. The underlying algorithms may either be based on an equation-error (EE) formulation or an output error (OE) formulation. Since the equation-error formulation is a convex problem with one global minimum and due to its straight forward design it is often used in practical implementations. The main disadvantage of the EE formulation is the fact that a general stability of the filter is only guaranteed in case of special filter constraints, which often results in a conflict with practical implementations.